The instructions for the extender should have instructions on how to connect to it once you have it set up, so have a look in there and see if it gives you any indications about which network to choose. If you have a laptop, you could try moving from the router to the extender and back with it and try and figure out what's what based on how the signal strength is increasing and decreasing.

Also, I don't know what the situation is like now with these extender products, but the last time I used one (a few years ago), it was a Linksys model and I'm pretty sure it was only compatible with Linksys routers, so double-check and make sure that the D-Link extender can actually work with your router.

Probably would of been best to buy two of the same routers which support DD-WRT. Then you wouldn't have any compatibility issues.

When I wanted to "extend" my wireless though, I used an older router, disabled DHCP on it and setup the Wireless stuff to the same settings as my main AP. Then I hooked up an Ethernet cable to the LAN1 port back to my main router.